The space shuttle is now a proven space transportation system in which space crews will use the spacecraft orbiter again and again in launches from the earth. The space shuttle system is composed of the orbiter with its main rocket engines, which carries the crew and payload, a large external tank that contains the propellants for the orbiter's main engines, and two solid rocket boosters. The orbiter and boosters are reuseable, but the external tank is expended on each launch.
The space shuttle during ascent has the large cylindrical external tank at its center, the two cylindrical solid rocket boosters on opposite sides of the external tank, and the orbiter parallel to the third side of the external tank. The external tank is composed of a forward pressure vessel for cold liquid oxygen and a larger aft pressure vessel for cold liquid hydrogen and the two pressure vessels are joined together by a rather heavy cylindrical intertank or intervessel structure.
The two solid rocket boosters are attached to the external tank so the thrust therefrom is transmitted to the cylindrical intertank structure from two forward attaches. The aft attaches of the solid rocket boosters are hinged to the rear wall of the external tank so as to provide lateral rigidity only and do not transmit the thrust of the solid rocket motors.
The orbiter's forward attaches to the external tank are to the cylindrical intertank structure and are hinged for lateral rigidity only so as not to receive the thrust from the orbiter's main engines. It is the aft attach of the orbiter to the external tank that transmits the thrust of the orbiter's main engine, which results in the main engine thrust transversing the comparatively light pressure vessel for hydrogen before reaching the forward heavy pressure vessel for oxygen.
During ascent of the space shuttle, the solid rocket boosters following their depletion are explosively severed and eight small solid rocket motors move the boosters away from the external tank. A parachute is deployed from each booster so as to allow late recovery. Later during the launch the orbiter's attaches to the external tank are explosively severed to allow the external tank to be discarded. It is recognized that the external tank could be allowed to go into earth orbit with the space shuttle.
The indirect routing of the thrust loads from the orbiter and solid rocket boosters to the external tank requires the use of stiffening rings and frames which make the external tank unnecessarily heavy. The fact that the forward oxygen pressure vessel is the heaviest part of the external tank and cantilevered from the lighter aft hydrogen pressure vessel, requires that the solid rocket boosters thrust enter at only the forward attaches to the intertank structure. The intertank structure is heavy because it has to distribute the booster thrust from only two points. The large diameter of the external tank places the orbiter and solid rocket boosters far from the center line of the external tank and causes high aerodynamic drag losses during ascent.